Door alarm circuit

At the same time C1 will discharge change R1 , when discharge be finished. It make voltage at a pin B of Q1 be valuable a little make Q1 and Q2 stop work. And have current flow through R1 reach C1 again. The work in this character will cause the loud sound goes up. But when shut the door make switch S1 pressed. Q1 can not can work. Because voltage at a pin B be 0V make the circuit doesn’t work and have no the sound comes out from a loudspeaker.

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